1. Field of the Invention
In a mobile communication system employing a CDMA (Code Division Multiple Access) scheme, the present invention relates to a mobile station apparatus for carrying out a service carrier detecting process and an out-of-zone determining process in the mobile station apparatus, a mobile communication system, and a carrier detecting method.
2. Related Background Art
In a mobile communication system using a CDMA scheme, it is necessary for a mobile station apparatus to enable communications with a base station in charge of the in-zone cell immediately after being energized or moved, in order to obtain information required for the mobile station apparatus to shift to an in-zone standby state from the notice information transmitted by a service carrier. For enabling communications with the base station, the mobile station apparatus establishes synchronization with the base station, and specifies a scrambling code of the channel through which the base station transmits the notice information. Such a series of operations carried out by the mobile station apparatus are referred to as cell search in general.
Operations of a cell search will now be explained in brief. First, spread spectrum signals are received at a frequency of the service carrier, and a signal having traveled a specific channel (PSCH) among thus received spread spectrum signals is fed into a matched filter, whereby a slot timing is detected. Subsequently, using thus detected slot timing and a specific channel (S.SCH), a frame timing and a scrambling code group are specified. Using this scrambling code group and a specific channel (CPICH), a scrambling code is specified. Further, a signal having traveled a specific channel (BCCH) is subjected to reverse-spreading by use of the specified scrambling code, whereby cell-specific information reported to all the mobile station apparatus within the cell is acquired. According to this information, the mobile station apparatus shifts to the in-zone standby state.
In cases where no information can be acquired, no scrambling code can be specified, or no synchronization can be detected after carrying out a series of cell search operations, the mobile station apparatus turns back the cell search procedure to the slot timing detection, and intermittently repeats a series of operations until the information is acquired upon reverse-spreading. If it is determined that the frequency subjected to the cell search differs from the frequency at which the base station transmits a signal, a cell search will be carried out at another frequency. Here, the mobile station apparatus will be in an out-of-zone state during when information of the base station cannot be acquired upon reverse-spreading.
When the frequency of a service carrier has not been specified in the mobile station apparatus, it must be specified before carrying out a cell search. This operation of the mobile station apparatus for specifying the frequency of a service carrier is referred to as carrier search.
In conventional mobile communication systems using CDMA schemes, mobile station apparatus have detected service carriers by executing cell searches at all the frequencies where carriers specified by the systems can be set.
Meanwhile, the specification of FDD scheme proposed by a group known as 3GPP (3rd Generation Partnership Project) requires that frequencies capable of setting carriers be arranged at frequency intervals of 200 kHz within a bandwidth of 60 MHz for each of uplinks and downlinks. Therefore, when one wishes to use a roaming service abroad from another provider, for example, it may take a considerable time to detect a service carrier used by this provider, and the power consumption required for a cell search may become enormous.
If roaming services between providers are limited while the number of kinds of carrier frequencies at which the mobile station apparatus can receive the services is very small as have been heretofore, cell searches may be carried out sequentially at these service carrier frequencies kept beforehand in a storage device of the mobile station apparatus without carrying out carrier searches in particular, so as to detect service carriers.